Panels used in the multi-chip module and flat panel display industry are thin, the thickness being much the same as that of a microscope slide (a panel thickness is about 0.028 inch to about 0.125 inch, typically). But they are vastly larger than a microscope slide, one size being 400 mm by 400 mm. The substances of which the large thin panels are made may vary, and include glass, metal and circuit-board material.
The panels must be flat during each photolithographic exposure, but in their free (unconstrained) conditions are very seldom flat. Not only are they not flat, but they are irregularly not flat--as distinguished from, for example, bowed. If the panels were not held flat during exposure, the results would be poor resolution, and distortion, etc.
Even if the panels were inherently or initially flat, they very often become warped during processing. The coating of photoresist onto the front sides of the panels, followed by removal of unexposed (or exposed) areas, introduces stresses that cause the warping.
There are major factors, in addition to the thinness and the large sizes of the warped panels, that make it difficult to make them flat and/or to hold them flat. One is that the panels must not be handled from the front, but only from the back. The reason is that there is the photoresist on the front, which must not be scratched and/or disturbed, or so contacted as to cause cross-contamination between panels.
Another of the major factors is that the panels must not only be constrained in flat condition, but so constrained in an exact, precisely-predetermined exposure plane. If this were not so, the photolithographic exposure would not achieve the necessary high-precision results.
Thus, the system must automatically (1) handle the large, thin, warped panels from the back only, and (2) make them locate not only flat but (3) locate in a precisely-predetermined exposure plane.
It may be thought that it would be enough to snub the warped panels against three registration (locator) pins, and then apply a large amount of suction adjacent the flat surface of the chuck having the usual vacuum (suction) grooves. This is not so. It would not be a good thing to suck a major volume of air from behind a warped panel, in an attempt to overcome the big leakages resulting from the warping. To do so would typically require wide vacuum grooves that could bend the panel somewhat into them so that it would not be flat. It would also require large vacuum hoses or tubes, which are undesirable in systems where the chuck is moving. It would also necessitate ignoring the fact that panel edges can hang up on the pins, especially after snubbing.
It is pointed out that in order to expose very large panels the chuck is not only moving but is preferably at a major angle to horizontal. Such an angle provides major benefits, but also tends to increase the degree of warping, and to increase the need for always having full control of the panel so that is does not drop or tip.
Thus, because of the added factors stated in the two preceding paragraphs, additional requirements for a fully satisfactory system include (4) elimination of any necessity for large suction with attendant big grooves and hoses, and (5) provision of effective elements for handling each panel at all times, without ever interfering with desired movements of the panel, and (6) provision of devices for preventing the panels from hanging up on the registration pins.
There are additional factors relative to the achievement of a satisfactory and practical apparatus and method of the present type. One is simplicity, such as results from performing plural functions with simple elements and simple controls.